Northern leaf blight (NLB), induced by the fungal pathogen Exserohilum turcicum (previously called Helminthosporium turcicum), is a serious foliar wilt disease of maize in many tropical and temperate environments. Symptoms can range from cigar-shaped lesions on the lower leaves to complete destruction of the foliage, thereby reducing the amount of leaf surface area available for photosynthesis. A reduction in photosynthetic capability leads to a lack of carbohydrates needed for grain fill, which impacts grain yield. Mid-altitude regions of the tropics, about 900-1600 m above sea level, have a particularly favorable climate for northern leaf blight, as dew periods are long and temperatures moderate. However, northern leaf blight can also yield losses of 30-50% in temperate environments, such as in the United States, during wet seasons, particularly if the infection is established on the upper leaves of the plant by the silking stage.
The fungus Exserohilum turcicum (Et) overwinters as mycelia and conidia on maize residues left on the soil surface. The conidia are transformed into resting spores, and during warm, moist weather, new conidia are produced and then carried by wind or rain to lower leaves of young maize plants. Infection requires the presence of water on the leaf surface for 6-18 hours and a temperature of between 66 and 80° F. If infection occurs, lesions develop within 7-12 days and produce new conidia, which spread the infection to secondary sites. Disease management strategies include crop rotation, destruction of old maize residues by tillage, and fungicide application, all of which are aimed at reducing the fungal inoculum. However, the most effective and most preferred method of control for northern leaf blight is the planting of resistant hybrids.
Several varieties or races of Exserohilum turcicum are present in nature, leaving growers with two hybrid options: partial resistant hybrids, which offer low-level, broad spectrum protection against multiple races, and race-specific resistant hybrids, which protect against a specific race. Genetic sources of resistance to Exserohilum turcicum have been described, and four Exserohilum turcicum (previously called Helminthosporium turcicum) resistance loci have been identified: Ht1, Ht2, Ht3, and Htn1. Gene Ht1 maps to the long arm of chromosome 2 where it is closely linked to umc36 (Coe, E. H. et al. (1988), Corn and Corn Improvement, 3rd edn., pp. 81-258), sgcr506 (Gupta, M. et al. (1989) Maize Genet. Coop. Newsl. 63, 112), umc150B (Bentolila, S. et al. (1991) Theor. Appl. Genet., 82:393-398), and pic18a (Collins et al. (1998) Molecular Plant-Microbe Interactions, 11:968-978), and it is closely flanked by umc22 and umc122 (Li et al. (1998) Hereditas, 129:101-106). Gene Ht2 maps to the long arm of chromosome 8 in the umc48-umc89 interval (Zaitlin et al. (1992) Maize Genet. Coop. Newsl., 66, 69-70), and gene Ht3 maps to chromosome 7 near bnlg1666 (Van Staden, D et al. (2001) Maize Genetics Conference Abstracts 43:P134). The Htn1 gene maps to chromosome 8, approximately 10 cM distal to Ht2 and 0.8 cM distal to the RFLP marker umc117 (Simcox and Bennetzen (1993) Maize Genet. Coop. Newl. 67, 118-119; Simcox and Bennetzen (1993) Phytopathology, 83:1326-1330; Chung et al. (2010) Theor App Gen Epub).
The methods of controlling northern leaf blight by reducing fungal inoculum require additional time and resources on the part of the farmer, and in addition, can have detrimental effects on the environment. This makes the planting of resistant hybrids even more attractive to farmers and the general public. Thus, it is desirable to provide compositions and methods for identifying and selecting maize plants with enhanced resistance to northern leaf blight. These plants can be used in breeding programs to generate high-yielding hybrids with resistance to northern leaf blight.